For starters, I play a hybrid TB.

More and more matches I'm dealing with seem to have oaks.  Last match I played, the other team had Oak, Sedna, QoT, and Rook.  Our team consisted of me (TB), Regulus, QoT, and UB.  At the start, UB and I had some pretty good caps.  The QoT was sheilding us periodically, and the Reg was, well, doing his thing.  (I really don't like having reg players on my team, no offense to the good ones.  Most just don't play a team game, and it seems like that DG slot would be better filled. =/)  Anyway, I didn't see but fleeting glimpses of the Oak for the first 6 levels or so.  It was just the QoT and Sedna running around together capping flags with mad shielding/heals (so they're basically unkillable).  Rook was pushing one lane at a time, though I kept making him switch via harrassment.  By level 7, Oak showed up with QoT and Sedna, making for one hell of a team.  They just weren't killable.  About 3 of us would focus on one of them only for them to be shielded and instantly healed.  They literally tore down all our defenses to our base by me reaching level 8 (I was the first, being a primarily AoE TB, besides Fireball).  We managed to run them off (probably because they were all oom).  Then the Oak came back shortly after all beefed up and the same level as me.  Two of my teammates were dead (not to be mean, but yeah, my team wasn't the best team), so Regulus and I tried to get Oak down, which should've been pathetically easy.  When he was at 200 health, he pulled off that oh-so-overpowered Invincible+HoL combo, which made me furious enough, but after that, the guy had a health pot.  By the time we exhausted his health 3 times (took 2 crystal hugs for me), the Sedna and QoT are back, which is just around the time that my teammates respawn.  Then, the rook is bashing in from another lane.  After thoroughly raping us a bit more, they add giants in to the mix.  We weren't outleveled or anything, but it felt like facing opponents a good 5 levels above me.

I know that was a pretty big wall of text, but someone please tell me something I could've done in that situation.  All of them weren't killable alone, much less in the zerg they had.  There probably wasn't anything I could do, considering nearly all of my team liked to wander off on their own.

TLDR: Oak+Sedna+QoT together=Overpowered against nearly anything.